Preheat the oven to 200°C/fan oven 180°C/ Gas Mark 6.
Mix together the apples, blackberries and caster sugar, then tip them into a 1 litre (1 ½ pint) ovenproof dish.
Put the flour, salt and porridge oats into a mixing bowl and add the TREX, rubbing it in with your fingertips until the mixture looks like crumbs. Stir in the demerara sugar, then sprinkle the topping evenly over the fruit.
Bake in the oven for 30-35 minutes, until the fruit is tender and the topping is golden brown.